In an FA (factory automation) network system, for the purpose of wire-saving, a single master device and a plurality of slave devices are connected with a daisy chain in some cases. In the system with a daisy-chain connection, communication between a first device and a second device that are not adjacent to each other is performed by relaying the communication by a device positioned between the first device and the second device.
The master device and the slave devices communicate with each other repeatedly in the same cycle. The mater device transmits command data to the slave devices. The slave devices transmit response data to the master device. As the communication time of the command data and the response data becomes shorter, the master device and the slave devices can exchange these command data and response data more closely and frequently. Therefore, the system can improve the control accuracy.
As a method for reducing the communication time, it is common to reduce the volume of communication data, or reduce the number of frames needed for communication. In the method for reducing the number of frames needed for communication, the volume of data needed for each frame can be reduced. Therefore, the communication time is reduced. As the data needed for each frame, a preamble that indicates the start of a frame, or a frame check sequence (FCS) that is a code to be added for performing error detection and correction on the frame, is exemplified.
As a related technique, Patent Literature 1 has described a communication system in which a master node and a plurality of terminal nodes are connected with a daisy chain. In this system, each of the terminal nodes relays and forwards the control data, transmitted from the master node, to the subsequent downstream node after the terminal node deletes a command field for its own node in the control data, and then shifts the second and subsequent command fields one by one to shorten the control data (paragraph [0029]).
Further, Patent Literature 2 has described a system in which a controller and a plurality of servo amplifiers are connected in a line. In this system, each of the amplifiers receives a frame from a higher amplifier, which includes command-data areas transmitted from the controller to the amplifiers, and then transmits the frame, from which a command-data area addressed to the amplifier of its own has been removed, to a lower amplifier (paragraphs [0039] to [0040]).
Furthermore, Patent Literature 3 has described a system in which a numerical control device and a plurality of amplifiers are connected with a daisy chain. In this system, each of the amplifiers transmits a transmission-start code to an upstream amplifier, and then transmits its own amplifier data, and subsequently the downstream amplifier data, received from the downstream amplifier, to the upstream amplifier (paragraph [0013]). When each of the amplifiers cannot receive the downstream amplifier data before it has completely transmitted its own amplifier data, the amplifier transmits idle-time data to the upstream amplifier after the transmission of its own amplifier data and until the reception of the amplifier data from the downstream amplifier, and thereafter transmits this downstream amplifier data to the upstream amplifier (paragraph [0028]).